1. Field of the Invention
The present invention relates to display devices for displaying images and, more particularly, to a thin flexible display device adapted to display images utilizing travel of fine particles in a gaseous phase between electrodes.
2. Description of the Related Art
Recently, an electrophoretic display device has been proposed which is designed to display images based on migration of electrophoretically migratory particles between a pair of opposite electrodes in a liquid phase provided between the electrodes (see patent document 1 for example). Since such an electrophoretic display device uses fine particles to display images, the display device can have a thin and flexible structure.
Such an electrophoretic display device, however, involves a problem of low responsiveness due to high resistance of the liquid to migration of migratory particles in the liquid phase. With a view to improving the response speed, there has been proposed a display device designed to display images by causing particles to travel in a gaseous phase provided between a pair of opposite electrodes. Such a display device in which particles travel in a gaseous phase can offer a higher response speed than the electrophoretic display device. Presently, the response speed of particles in the electrophoretic display device is about 100 msec, whereas that of particles in a display device of the type utilizing travel of the particles in a gaseous phase is 1 msec or less.
Examples of display devices of the type utilizing travel of particles in a gaseous phase for displaying images as described above include devices disclosed in patent documents 2 and 3. FIG. 1A is a schematic view illustrating the construction of the prior art display device disclosed in patent document 2 and the black display operation of the prior art display device, while FIG. 1B is a schematic view illustrating the construction of the same prior art display device and the white display operation thereof. As shown in FIGS. 1A and 1B, the prior art display device includes an image display medium having a first substrate allowing light to pass therethrough disposed on the viewer side, and a second substrate 21 disposed opposite to the first substrate 20. The first and second substrates 20 and 21 have their respective inner surfaces each formed with electrode 22 or 23 and charge transport layer 24 or 25 sequentially. Positively charged black particles 26 and negatively charged white particles 27 are encapsulated in the space between the first substrate 20 and the second substrate 21.
In the prior art display device thus constructed, voltage is applied across the electrodes 22 and 23 in accordance with an image to be displayed. Here, a voltage applied in the black display operation and that applied in the white display operation have opposite polarities. Referring first to FIG. 1A, description is made of the black display operation of the prior art display device. First, a voltage is applied across the electrodes 22 and 23 from an electric power source, so that the electrodes 22 and 23 become a negative electrode and a positive electrode, respectively. An electric field resulting between the electrodes 22 and 23 causes black particles 26 and white particles 27 present between the electrodes 22 and 23 to travel in respective directions by Coulomb force. In this case, positively charged black particles 26 travel toward the negative electrode 22, whereas negatively charged white particles 27 travel toward the positive electrode 23. In this way, black particles 26 are collected on the first substrate 20 side, while white particles 27 collected on the second substrate 21 side. In this state, when the viewer views the display device from the first substrate 20 side, black display is observed. In the white display operation, on the other hand, the electrodes 22 and 23 are applied with a voltage having a polarity opposite to the polarity of the voltage applied in the black display operation, as shown in FIG. 1B, whereby the electrode 22 and the electrode 23 become a positive electrode and a negative electrode, respectively. In this case, positively charged black particles 26 travel toward the negative electrode 23, whereas negatively charged white particles 27 travel toward the positive electrode 22. In this way, black particles 26 are collected on the second substrate 21 side, while white particles 27 collected on the first substrate 20 side. In this state, when the viewer views the display device from the first substrate 20 side, white display is observed. The principle described above makes it possible to display a desired image.
Non-patent document 1 discloses a display device as described below. FIG. 2A illustrates the sectional structure of one pixel portion of the electrophoretic display device disclosed in non-patent document 1 in the white display operation, while FIG. 2B illustrate the sectional structure of the same pixel portion in the black display operation. As shown in FIGS. 2A and 2B, a first substrate 28 allowing light to pass therethrough located on the viewer side and a second substrate 29 are disposed opposite to each other. The first substrate 28 has a concave curved surface on the side facing the second substrate 29 and hence functions as a concave lens. An electrode 34 allowing light to pass therethrough is formed on the first substrate 28 conformally to the concave curved surface. On the other hand, the second substrate 29 has a flat surface on the side facing the first substrate 28, the flat surface being formed with a rectangular electrode 31 in a predetermined region thereof. A black-colored wall 30 is formed on the second substrate 29 so as to surround the electrode 31. A transparent solution 33 is encapsulated in the space defined by the concave curved surface of the first substrate 29 and the colored wall 30. In the solution 33 are dispersed white migratory particles (hereinafter referred to as “white particles 32”) charged either positively or negatively.
The displaying operation of the electrophoretic display device thus constructed is described below. When a voltage is applied across the electrodes 31 and 34, white particles 32 migrate in the transparent solution 33 by Coulomb force. In the white display operation, white particles 32 migrate toward the electrode 34 to cover the concave curved surface side of the first substrate 28, as shown in FIG. 2A. In the black display operation, on the other hand, white particles 32 migrate toward the second substrate 29 and adhere to the surface of the electrode 31, as shown in FIG. 2B. In the latter case, light becoming incident on the device from the first substrate 28 side is refracted and scattered by the concave curved surface of the first substrate 28 functioning as a concave lens, thereby to illuminate the surface of the colored wall 30 selectively. Thus, the color of a region of the colored wall 30 illuminated with light (black in this case) participates in display, whereas white particles 32 adhering to the electrode 31 not illuminated with light fail to participate in display. In such a construction, since a single kind of particles 32 migrate in the transparent solution 33, particles 32 can migrate without obstruction by other kind of particles, hence, rapidly. Accordingly, the response speed can be improved. Further, since the inner surface of the first substrate 28 serves as a concave lens as described above, the electrode 31 to which white particles 32 are adhering can be shown smaller in the black display state. Therefore, the device can offer improved contrast.